A rare earth magnet using a rare earth element is also called a permanent magnet and is used for a motor making up a hard disk or an MRI as well as for a driving motor of a hybrid vehicle, an electric vehicle, etc.,
The index indicative of the magnet performance of the rare earth magnet includes residual magnetization (residual flux density) and coercive force. Meanwhile, as the amount of heat generation grows due to the trend to a more compact motor and a higher current density, heat resistance is more increasingly required also of the rare earth magnet used therein, and how the coercive force of a magnet can be maintained in use at high temperatures is one of important research themes in this technical field. Considering an Nd—Fe—B magnet that is one of rare earth magnets often used in a vehicle driving motor, attempts are made to increase the coercive force, for example, by achieving refinement of a crystal grain, using an alloy of a composition having a large Nd amount, or adding a heavy rare earth element having a high coercivity performance, such as Dy and Tb.
As the rare earth element, there are not only a general sintered magnet in which the crystal grain constituting the structure is on a scale of approximately from 3 to 5 μm, but also a nanocrystalline magnet in which the crystal grain is refined to a nanoscale of 50 to 300 nm.
The microstructure of an Nd—Fe—B general rare earth magnet consists of an Nd-rich crystal grain and a grain boundary intervening between crystal grains. Since Nd constituting the crystal grain is an expensive rare earth element, how the amount of the element used can be reduced while ensuring the magnet performance is one of important development challenges in this technical field.
As the measure regarding the reduction in the amount of Nd used, it is conceivable to use a light rare earth element such as Ce and La or use an element such as Gd, Y, Sc, Sm and Lu.
However, as well as in the case of applying such an element in place of Nd, even when most of Nd is substituted by such an element, significant deterioration of the magnetic properties of the rare earth magnet is envisaged. Therefore, the amount of such an element used must be limited, and an effect of sufficiently reducing the material cost cannot be expected. Furthermore, when such an element having low magnetic properties is used, there is generally a very strong tendency that the use form thereof is limited to an isotropic form.
In the case where anisotropization of a rare earth magnet using the above-described light rare earth element or an element such as Gd and Y is attempted, the coercive force of the rare earth magnet decreases significantly, for example, in the working process such as hot plastic working, and the magnetic properties are inevitably deteriorated.
Here, Patent Document 1 discloses a magnetic material produced through a rapid solidification process and the subsequent heat annealing process, wherein the magnetic material has, by atomic percentage, the following composition: (R1-aR′a)uFe100-u-v-w-x-yCovMwTxBy (wherein R is Nd, Pr, didymium (a natural mixture of Nd and Pr, having a composition of Nd0.75Pr0.25), or a combination thereof, R′ is La, Ce, Y, or a combination thereof, M is one or more of Zr, Nb, Ti, Cr, V, Mo, W and Hf, T is one or more of Al, Mn, Cu and Si, 0.01≤a≤0.8, 7≤u≤13, 0≤v≤20, 0.01≤w≤1, 0.1≤x≤5, and 4≤y≤12) and exhibits a residual magnetism (Br) value of about 6.5 kG to about 8.5 kG and an intrinsic coercive force of about 6.0 kOe to about 9.9 kOe.
While the magnetic material disclosed is a magnetic material where part of Nd is substituted by La or Ce, this is a compositional material for a rare earth-lean nano-composite magnet or a compositional material close thereto, and such a compositional material is composed of not an anisotropic but isotropic magnetic powder. Because, in the case of a compositional material for a nano-composite magnet or a compositional material close thereto, even when hot plastic working is performed in the plastic state with an attempt to form an oriented magnet, only a magnet having insufficient magnet performance can be formed.
In this way, the magnet material disclosed in Patent Document 1 may be an isotropic magnet material and is improper as a magnet material for the manufacture of an anisotropic rare earth magnet. Furthermore, Patent Document 1 is absolutely silent as to taking a measure for imparting anisotropy to such an isotropic magnet material.